The invention relates generally to turbomachines such as steam turbines. More particularly, the invention relates to a shell differential expansion detector for detecting axial differential expansion between an inner and outer shell of the turbomachine.
Turbomachines such as gas and steam turbines typically include a centrally-disposed rotor that rotates within a stator. A working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially outwardly from the rotor shaft. The fluid imparts energy to the shaft, which is used to drive a load such as an electric generator, compressor, or pump.
Steam turbines in particular are typically divided into sections based on the applied cycle, steam conditions, and mechanical design constraints for a particular application. In general, a steam turbine unit includes one or more sections. The steam conditions for each section vary, resulting in high-, intermediate- (no reheat), reheat-, and low-pressure sections. Particularly as higher pressure and temperature steam conditions have been applied to these sections, double shell construction is desirable to contain the steam within the appropriate section.
Mechanical constraints require that the inner and outer shells be connected to one another at an axial position along their longitudinal extent. The connection point may also be called a fit. During operation, the interior of the turbine is heated and cooled by convection from the steam at its surface, radiation from other adjacent hot surfaces, and conduction within the shells during operation of the turbomachine. Typically the rotor heats first, resulting in expansion of the rotor, known as a long rotor condition. Steam then heats the inner shell and finally the outer shell, causing expansion of the respective shell members. Heating and cooling of the inner and outer shells changes with time; steam conditions including pressure, temperature, and flow; turbomachine load; and other factors. As a result, the axial expansion undergone by the outer and inner shells may vary significantly throughout the operation of the turbomachine.
In order to prevent axial rubbing of the stationary and rotating parts of the turbomachine over the variety of expanded and contracted positions, and to provide sealing of the stages within the sections and regions where the rotor protrudes through the inner and outer shells into the outside environment, the axial differential expansion of the rotor relative to the outer shell may be measured. However, this measurement fails to provide a direct measurement of the relative expansion of the rotor and the inner shell, where the axial clearance between the rotating and stationary parts is established. This lack of direct measurement of inner shell expansion may lead to uncertainty in the differential clearances and compromise stage and shaft sealing.